On body e-shaped patch antenna for biomedical application

Received May 31, 2018 Revised Dec 14, 2018 Accepted Jan 11, 2019 An E-shaped micro strip patch antenna is designed and analyzed in this paper which operates in MICS (402.0-406.0MHz) band. The Performance has been observed on a body of human phantom model as well as in free space with different conducting material for the designed antenna. The height of this antenna is taken 1.5mm from the ground plane. At resonance frequency of 405 MHz the S11 parameter is obtained in free space is -23.26dB for conducting material of aluminum and -17.96dB is measured on human phantom body at 405 MHz of resonance frequency. FR4 is used as substrate material. The Specific Absorption Rate (SAR) is found to be 0.3562 W/kg by placing the antanna on human phantom model. VSWR and directivity has been analyzed also.


INTRODUCTION
The design of implantable patch antennas becoming more popular for dealing with issues related to biocompatibility, miniaturization and patient safety [1].Peoples are now depending up on implantable medical devices for improving the quality of lives [2].Furthermore, prototype fabrication of miniature antenna structures is highly challenging [3].Biomedical sensors have also put a well remark on this field to operate successfully [4].
Recenty implantable antennas are use in drug pumps and nerve signal processing [5].Wireless transmission is most commonly performed in the 402-405 MHz frequency band, which has been exclusively allocated for medical implant communications systems (MICSs), is internationally available and feasible with low-power circuits [6].In small antenna theory, as the radius of sphere including the full antenna structure increases,thelimitation of radiation efficiency increases [7].To increase the performance of a microstrip patch antenna there are several methods like increasing the thickness of substrate, different impedance matching and feeding techniques [8].It is challenging to design an antenna to operate in human tissue considering size, bandwidth, radiation efficiency, operating frequency, SAR and VSWR [9].
This paper proposed a patch antenna, whish is designed for appling medical sectors.A phantom muscel model is created according to dielectric properties of human body for performance analysis of this designed antenna.Performance is observed by placing the antenna on a phantom muscesl model and also in free space.During placing the antenna on a body model, silicon gel is used for safety.S11 (Return loss) and VSWR are observed by changing different conducting material.Radiation pattern and antenna performance in far field region is also observed.SAR is also measured for ensuring the safety in on-body applications.

RESEARCH METHOD
Ground layer, substrate layer and the microstrip patch layer is used to design this antenna in free space.To get the MICS frequency band, the antenna is optimized by length, width, and also slot size.

Figure 1. Designed E-shaped Antenna
The patch of the E shape antenna is designed width of 36mm and the length is 32mm.Each layer thickness is designed with 0.5mm.For designing the antenna FR-4 substrate material is used.The patch material is considered as copper.Length and width were determined by equations of references forpatch of the antenna [10].

RESULTS AND ANALYSIS
In CST microwave studio the input impedance is considered 50 ohms for observing the antenna performance.The initial frequency was taken from 0 to 2GHz.

Simulated Result for E-shaped Antenna (free space)
In free space, S11 parameter is observed for designed E-shaped micro strip patch antenna, which is shown in Figure 2. The value of S11 is obtained -23.26dB, -17.32dB and -17.87dB respectively for conducting material of aluminum, copper and gold.505 MHz is the middle frequency of the (400-600 MHz) band and in this point, there is found to the minimum bandwidth of the total loss of human body [11].The polar view of the E-shaped antenna is observed in Figure 3. Main lobe of an antenna means the maximum power containing of this lobe.There are also side lobe of the antenna.Here is shown the polar view and main lobe.The main lobe magnitude is found 1.47 dBi and the direction is 110.4 degree.

Figure 3. Polar view of E-shaped antenna (free space)
The radiation pattern is defined as a graphical representation of the radiation properties [12].Far field region of antenna is dominated by radiated fields, with the E-and H -fields orthogonal to each other [13].( 1 )

Figures-3 and
VSWR is always a real and positive number for antennas [8].

Simulated E-shaped Antenna in Phantom Model
A Human Phantom Mascel Model has been created with Skin 4mm, Fat 4mm and the muscle 8mm.The antenna is implanted on that model for observing the performance at bio environment is shown in Figure 6.After implanting the antenna on phantom tissue, the S11 parameter is measured -17.96dB at resonance frequency, which ensure the biocompatibility of designed antenna is shown in Figure 7.In this analysis the resonance frequency is found 0.4046 GHz, which lies in MICS band.On body e-shaped patch antenna for biomedical application (Raja Rashidul Hasan) 155 S11 parameter is also known as the reflection co-efficient [14].Bandwidth of the antenna can be measured at -10dB by taking a line.From Figure 7 the bandwidth of this antenna is measured 8MHz (402MHz -410MHz).Figure 8 shows the 3D view of far-field region of the E-shaped antenna after placing on human muscle model.Here the antenna directivity is observed 1.871 dBi.The total efficiency is found -35.75 dBi.For all the conducting material, far-field region was observed.The directivity for other conducting materials is closed to this result.
In Figure 9 the VSWR is also observed on human tissue and found to be 1.3591 at resonant frequency of 405 MHz, which is good for impedance matching.Specific Absorption Rate (SAR) is determine the rate of energy absorption by human body, when RF energy is exposed.According to IEEE C95.3-2002 standard and also FCC and ICNIRP guidelines, SAR should not be exceeding 1.6 W/kg for average 1-g of tissue [15].In Figure 10, the SAR is measured 0.3563 W/kg for 1 gram of tissue, when the antenna is placed on human muscel model.From this observation we can say that, the radiation of antenna wil not be harmful for on body applications.

Figure 2 .
Figure 2. S11 parameter of E-shape antenna in free space

Figure- 4
shows the radiation pattern of polar view and 3D view.The directivity and total radiation efficiency of this antenna is measured 1.688 dBi and -35.61 dB respectively.

Figure 4 .
Figure 4. 3D view of Far-Field Region of the E-shaped antenna (free space)

Figure 6 .
Figure 6.E-shaped Antenna placed on human phantom mascel model

Figure 8 .
Figure 8. Far-field Radiation Pattern (3D view) on phantom muscle model

Table 1 .
Antenna Size Parameter